Cell phones contain a mix of electronic components, including image sensors that may or may not contain on-chip digital image processors, high-performance digital processors, memory circuits, active-matrix displays, power supply components including battery chargers, clock and frequency synthesis circuits, and digital radio transceivers. All these components may generate radio-frequency emissions, and, because of the compact nature of cell phones, are of necessity located in close proximity to at least one antenna and components of the receiver side of the digital radio transceivers. These digital radio transceivers may include transceivers operable from 460 to 6000 MHz to cover an assortment of base cell phone, LTE (long term evolution) cell phone, Bluetooth, Bluetooth low energy, IEEE 802.11 (WiFi) and other digital radio standards. These radio frequency emissions can couple through the antenna and directly to receiver side components to cause electromagnetic interference (EMI) with the digital radio transceivers.
Cell phone manufacturers and component manufactures need to understand how much of these radio frequency radiations (RFR) arises from each component of the cell phone, their cause, and how to reduce their coupling into the antennas and receiver side components that causes EMI to design phones less affected by EMI than competing phones. Phones less affected by EMI are more likely to have long range at low power where wanted signals to the digital radio transceivers are weak. It is important to not only understand RFR magnitude, but also frequency distribution since particular signals, such SCLK (an internal clock signal) of an image sensor, may produce significant EMI at both fundamental and multiple harmonics of their fundamental frequencies.
Improper design of sensor circuits, poor layout within the phone, poor shielding, poor antenna design, and other issues can allow RFR from sensors at such frequencies and their harmonics to jam digital radio communications attempted by the cell phone. For cell phone designs, EMI issues usually are reported at a very late stage of phone design by imaging sensor customers, usually right before the mass production of their products, demanding solutions in a very short time.
It is important that unavoidable RFR from the sensor IC itself be quantified separately from RFR from other portions of an imaging subsystem because solutions requiring internal changes to the sensor IC design are much more costly and take far longer to implement than solutions requiring modifications to printed circuit board layout and other components within the phone.
Sensor IC RFR must also be quantified with the sensor IC operating at full speed. Further, because radiated harmonics increase sharply as rise and fall times of signals decrease, sensor IC RFR must be characterized with low capacitive loading comparable to that found on signal lines internal to a cell phone, not the significantly greater loading typical of signal lines of general purpose IC test equipment.